The present invention relates to room temperature vulcanizable silicone rubber compositions and in particular the present invention relates to self-bonding two-package room temperature vulcanizable silicone rubber compositions.
Room temperature vulcanizable silicone rubber compositions are well known. Such silicone rubber compositions are divided in two types, the one-package room temperature vulcanizable silicone rubber composition and the two-package room temperature vulcanizable silicone rubber compositions.
The one-package room temperature vulcanizable silicone rubber compositions comprise compositions in which all the ingredients as well as the catalyst systems are mixed together and the resulting composition is stored in anhydrous state. As long as moisture does not come into contact with the composition, it does not cure. However, once the composition is applied to any surface or in some other application and thus comes into contact with moisture in the air or other types of moisture, it rapidly cures to a hard elastomeric state.
The two-package room temperature vulcanizable silicone rubber compositions are basically manufactured in two components, one being the base polymer and the other being the catalyst system. Each component is stored separately until it is desired to form the hard elastomeric rubber type of product. When it is desired to cure this system then the base polymer and the catalyst system are brought together and mixed, whereupon the resulting composition cures to the hard elastomeric state. Such a two-package room temperature vulcanizable silicone rubber composition is described, for instance, in Berridge U.S. Pat. No. 2,843,555.
Such two-package room temperature vulcanizable silicone rubber compositions have as ingredients a hydroxy-terminated diorganopolysiloxane mixed with an appropriate filler. This mixture forms the base polymer composition. The other part of the two-package system comprises an alkyl silicate or partially hydrolyzed alkyl silicate into which is added a curing catalyst selected from certain metallic salts of organic carboxylic acids such as lead or tin octoate. Such a composition has the main advantage that when the two components are mixed together, that is, the base polymer and the catalyst component, the composition can be molded and cured in place at room temperature. Such compositions have many advantages for forming various types of molded products and even for forming roof coatings as well as other applications. Such a system is less expensive then one-package room temperature vulcanizable silicone rubber compositions, as well as being in certain applications easier to work with. However, such a two-package system as described in the Berridge patent and further as described and developed up to the present time results in a cured silicone rubber composition which does not form a strong adhesive bond between the silicone rubber composition and the substrate to which it is applied. It can be appreciated that for such an application as a roof coating, it would be highly desirable to form a two-package room temperature vulcanizable silicone rubber composition which would form as strong a bond as possible between the silicone rubber layer and the substrate. It is most preferred that the bond be so strong between the silicone rubber layer and the substrate that upon the application of pressure or tension upon the rubber layer that there will be cohesive failure rather than adhesive failure.
The name cohesive failure is given to the case when upon the application of pressure to rubber or other material bonded to a substrate, the rubber ruptures or fails rather than the bond between the rubber and the substrate. The designation adhesive failure is given to the case when upon the application of pressure or tension to a rubber substrate laminate, the bond between the silicone rubber and the substrate fails or gives way before the rupturing of either the rubber layer or the substrate layer.
To obtain such a bond between two-package room temperature vulcanizable silicone rubber compositions and various substrates, there has come into use various primers. Such primers are applied on the substrate, allowed to dry and then the two-package silicone system is applied thereover and cured. One common example of such a primer system for two-package silicone rubber compositions comprises a silicone resin and an alkoxy silane dissolved in a mixture of various solvents. However, even with such a primer system there is not always obtained a sufficiently strong bond between the silicone rubber composition and the substrate. In fact, even with the use of a primer system as mentioned above, in many cases when such laminates are tested and tension is applied to the cured silicone rubber composition there is obtained adhesive failure rather than cohesive failure.
In addition, such primer systems and in particular the bond formed with such primer systems is hydrolytically unstable, i.e., the presence of water and particularly a great deal of water degrades the bond and the cohesive action of the primer system in bonding and joining the silicone rubber layer to the substrate. In the case where the two-package silicone rubber composition is to be applied as a roof coating, it can be seen that this hydrolytic instability is a great disadvantage. Such hydrolytic instability is the case with almost all known primer systems for two-package room temperature vulcanizable silicone rubber compositions. This hydrolytic instability which results in a rupturing or weakening of the bond between the silicone rubber composition and the substrate is also true with other types of silicone rubber compositions whether or not a primer is used. Thus, common types of one-package room temperature vulcanizable silicone rubber compositions which are applied to a substrate will normally not adhere or bond as strongly as desired to the substrate in the continual presence of moisture or water.
In the case of two-package room temperature vulcanizable silicone rubber compositions which are bonded to various substrates with or without the use of a primer, this hydrolytic instability is even more marked.
In the case where the silicone rubber composition is used as a coating in the construction of buildings and particularly as a roof coating, it can be appreciated that it is of paramount importance that the silicone rubber composition be bonded to the roof substrate so as to form a bond which is not hydrolytically unstable. As can be appreciated, roofs are constantly exposed to excessive moisture such as rain water, snow, ice and sleet and if the bond between the substrate and the silicone rubber composition is hydrolytically unstable then the bond will shortly rupture.
It is one object of the present invention to provide for a novel self-bonding room temperature vulcanizable silicone rubber composition.
It is another object of the present invention to provide a process for producing a novel self-bonding room temperature vulcanizable silicone rubber composition.
It is still another object of the present invention to provide a novel self-bonding room temperature vulcanizable silicone rubber composition which will form superior bonds with various metals, masonry and wood substrates.
It is an additional object of the present invention to provide a novel room temperature vulcanizable silicone rubber composition which forms bonds with various substrates whether of metal, plastic, masonry or wood wherein the bond formed with such substrates is hydrolytically stable.
These and other objects of the present invention are accomplished by means of the silicone rubber composition defined below.